Full of water and running is key - dry cement all around is too. If they 'just hosed down the patio' before you arrived, come back when it's dry as a bone. Same goes for any moisture in the motor compartment.
Be sure the tub is at temperature - and check that all systems work. Listen for noisy motors on jet pumps and circulation pumps.
Headrest pillows can be a pain to replace - especially on off brand tubs. If they claim that the pillows can be purchased and replaced easily, give them the opportunity to prove it.
8-)